This invention relates to semiconductor ceramic compositions for boundary layer capacitors. More particularly, it relates to semiconductor ceramic compositions which make it possible to produce boundary layer ceramic capacitors having high permittivity and high breakdown voltage characteristic at high percent non-defective.
It has been known that ceramic capacitors of a boundary layer type are made by forming an insulating layer on grain surfaces of crystals of strontium titanate semiconductor ceramics. These boundary layer ceramic capacitors have widely been used because of their large apparent permittivity, small temperature coefficient of permittivity and, small dielectric loss.
In order to produce boundary layer ceramic capacitors having large apparent permittivity, it is necessary to fire ceramic materials so that the crystals of the semiconductor ceramic will have a grain size ranging from 50.mu. to 100.mu.. For this reason, the ceramic materials are required to be fired in a neutral or reducing atmosphere during the firing process. Usually, increasing grain size of crystals and, increasing the reducing action of the atmosphere tends to increase partial welding of ceramic disks, which are piled, during the firing process. This can be inhibited by adding zirconia powder between piled ceramic disks. Even under such a condition, it is difficult with the conventional composition to produce semiconductor ceramic disks which do not weld together, and the percent non-defective thereof is about 70%. Thus, it is necessary to separate welded disks into individuals, resulting in the increase of the cost of ceramic capacitors.
It is therefore a main object of the present inventon to provide a semiconductor ceramic composition for boundary layer capacitors which makes it possible to produce boundary layer capacitors having high permittivity and high breakdown voltage characteristic at high percent non-defective.
According to the present invention, there is provided a semiconductor ceramic composition for boundary layer capacitors consisting essentially of 99.90 99.995 wt % of a semiconductor ceramic component consisting of strontium titanate or a modified strontium titanate solid solution and at least one semiconductorizing agent, and 0.005 to 0.1 wt % of phosphorus.
The above composition may further contain 0.015 to 0.3 wt % of copper. In such a case, a semiconductor ceramic composition of the present invention consists essentially of 99.6 to 99.98 wt % of a semiconductor ceramic component consisting of strontium titanate or a modified strontium titanate solid solution and at least one semiconductorizing agent, 0.005 to 0.1 wt % of phosphorus, and 0.015 to 0.3 wt % of copper.
In this specification, the strontium titanate or a modified strontium titanate solid solution means a solid solution having a composition expressed by the general formula: EQU (Sr.sub.1-x A.sub.x)(Ti.sub.1-y Zr.sub.y)O.sub.3
wherein A is Ba or Ca, x and y are mole fractions of the respective components and take respective values in the following range. 0.ltoreq.x.ltoreq.0.20, 0.ltoreq.y.ltoreq.0.20
At least one semiconductorizing agent, or doping element may be selected from the group consisting of Sb, Ta, Nb, W, Y, La, and rare earth elements. The semiconductorizing agent is incorporated into the strontium titanate or modified strontium titanate solid solution in an amount not more than 5 wt %. If the content of the semiconductorizing agent is more than 5 wt %, the resistivity of the ceramic is considerably increased, thus making it difficult to semiconducterize the ceramics.
Phosphorus contributes to prevent welding of ceramics during the firing process when it is contained in the ceramics in an amount within the range of 0.005 to 0.1 wt %. If the content of phosphorus is less than 0.005 wt % or more than 0.1 wt %, its effect is scarcely obtained.
Copper contributes to increase the breakdown voltage of the produced boundary layer capacitors when it is contained in the ceramics in an amount within the range of 0.015 to 0.3 wt %. If the content of copper is out of the above range, it is difficult to increase the breakdown voltage of the boundary layer capacitors.
According to the present invention, it is possible to produce boundary layer ceramic capacitors without welding of semiconductor ceramic disks during the firing in a neutral or reducing atmosphere.
When producing boundary layer capacitors, grain boundaries of crystals of the semiconductor ceramics are converted to the insulating state by heat-treating in an oxidizing atmosphere at a temperature of 1000 to 1300.degree. C. after applying paste containing at least one metal or metal oxide to the surfaces of the semiconductor ceramic disks. As at least one metal or metal oxide for converting the grain boundary of crystal to an insulating layer, there may be used those such as V, Cr, Mn, Fe, Co, Ni, As, Sb, Tl, Bi and their oxides. These metals or oxides diffuse into the semiconductor ceramic crystal by the heat-treatment. The proper amount of these metal or oxides to be applied to the semiconductor disk depends on their kind, but they enable the product to have a constant dielectric characteristics when applied in an amount of 1 to 4 wt % with respect to the weight of semiconductor ceramics. If the amount of these metal or oxides is out of the above range, the insulating resistance of the product is lowered and the dielectric loss is increased. As a method for applying the metal or its oxide to the surface of the respective disk, there may be employed a coating method, diping method, spray method, vapour deposition method and the like.